The present invention relates to a scanning optical system for printing apparatuses such as printers, facsimile machines, copy machines, or the like.
In a scanning optical system, a laser beam is deflected by a reflection type deflector such as a polygon mirror or a galvano-mirror. The deflected laser beam is then converged by an imaging optical system to form a beam spot on a surface of a photo-sensitive drum, or an object surface. The deflector deflects the laser beam so that the beam spot moves across the object surface in a main scanning direction at a constant speed. In the meantime, the laser beam is modulated ON/OFF in accordance with image information so that an image made up of a plurality of dots is formed on the object surface.
Recently, some scanning optical systems are arranged so as to deflect the laser beam twice by the deflector before directing the laser beam toward the object surface, which systems will be referred to hereinafter as double reflection type scanning optical systems. FIG. 1 schematically shows a side view of a conventional double reflection type scanning optical system 10 observed from the main scanning direction.
In order to deflect the laser beam twice, a pair of mirrors (first and second mirrors 12 and 13), each of which extends in parallel with the main scanning direction, are arranged near a polygon mirror 11 in parallel to the auxiliary scanning direction. The polygon mirror 11 is driven to revolve about an axis 11a. A laser beam incident on a reflection surface of the polygon mirror 11 is reflected toward the first mirror 12 and then to the second mirror 13. The second mirror 13 reflects the laser beam back to the reflection surface of the polygon mirror 11. Then, the laser beam is deflected by the polygon mirror 11 for the second time and travels through between the first and second mirrors 12 and 13 toward the object surface to be scanned via an imaging lens 14.
The inclination of the first and second mirrors 12 and 13 are adjusted such that the laser beam incident on the polygon mirror 11 for the second time (after being reflected by the first and second mirrors 12 and 13) is parallel with the laser beam striking the polygon mirror 11 for the first time when observed from the main scanning direction. Accordingly, the laser beam reflected by the polygon mirror 11 for the second time is also parallel with the laser beam reflected for the first time (when observed from the main scanning direction).
In many cases, light incident on the polygon mirror 11 includes not only the laser beam for forming the beam spot on the object surface, which will be referred to hereinafter as a regular beam, but also unwanted light fluxes. The unwanted light fluxes include, for example, light generated by diffraction that occurs as the laser beam passes through an aperture stop and/or flare generated as the laser beam passes through a collimator lens.
Such unwanted light fluxes travel in a vicinity of and in parallel with the regular beam. Most of the unwanted light fluxes are reflected by the polygon mirror 11 toward the first mirror 12 like the regular beam. The unwanted light fluxes are then partially reflected by the first mirror 12 toward the second mirror 13. The remaining pass by the first mirror 12 and travel directly toward the object surface as shown in FIG. 2, which is a top view of the scanning optical system shown in FIG. 1. Note that, in FIG. 2, an unwanted light flux that travels toward the object surface after being deflected by the polygon mirror 11 only once is indicated by solid lines, while the regular beam that is deflected twice is indicated by broken lines.
As with the regular beam, the unwanted light fluxes deflected twice by the polygon mirror 11 are scanned across the object surface. The object surface, however, will be scarcely exposed to these unwanted light fluxes because these unwanted light fluxes have much lower light intensity compared to the regular beam while being scanned over the object surface at the same scanning speed as the regular beam.
On the other hand, the unwanted light fluxes that pass by the first mirror 12 are scanned over the object surface at a much slower scanning speed compared to the regular beam. These unwanted light fluxes are deflected by the deflector only once before reaching the object surface. Therefore, the angle for which these unwanted light fluxes are deflected, and hence the scanning speed thereof is half of that of the regular beam. This low scanning speed allows the unwanted light fluxes to expose the object surface and thereby form a ghost image thereon.
Therefore, there is need for a double reflecting type scanning optical system that is capable of preventing an object surface from being exposed by unwanted light fluxes that are deflected only once by a deflector.